SDS: make sdscatfmt() faster by pre-allocating a bit.

src/sds.c

@@ -603,6 +603,10 @@ sds sdscatfmt(sds s, char const *fmt, ...) {
     long i;
     va_list ap;
 
+    /* To avoid continuous reallocations, let's start with a buffer that
+     * can hold at least two times the format string itself. It's not the
+     * best heuristic but seems to work in practice. */
+    s = sdsMakeRoomFor(s, initlen + strlen(fmt)*2);
     va_start(ap,fmt);
     f = fmt;    /* Next format specifier byte to process. */
     i = initlen; /* Position of the next byte to write to dest str. */